The present invention is directed to a composition and method for inhibiting the formation and deposition of coke on fluid transfer tubes during the elevated temperature processing of hydro carbons. Delayed coking is used for converting any type of reduced crude to cracking feedstock. These systems operate at temperatures of 600.degree. to 1300.degree. F. Coke deposition occurs when hydrocarbon liquids and vapors contact the hot metal surfaces of the processing equipment.
Due to the complex makeup of the hydrocarbon and the elevated temperatures and the contact with hot metallic surfaces, it is not entirely understood just what is occurring during processing. It is thought that the hydrocarbons undergo various changes through either chemical reactions and/or decomposition of various unstable components of the hydrocarbon. The undesired bi-products produced include coke, polymerized products, deposited impurities and the like. Whatever the undesired product that may be formed, reduced economies of the process is the result. If these deposits remain unchecked, heat transfer, throughput and overall productivity are detrimentally effected. Moreover, downtime is likely to be encountered due to the necessity of either replacing and/or of cleaning the affected parts of the processing system.
Carbon formation also erodes the metal of the system in two ways. The formation of catalytic coke causes the metal catalyst particle to be dislodged. This results in metal loss and ultimately metal failure at a rapid pace. The other erosive effect occurs when carbon particles enter the hydrocarbon stream and act as abrasives on the systems tube walls.
While the formation and type of undesired products vary as to the hydrocarbon being processed and the conditions of the processing, it may generally be stated that such products can be produced at temperatures as low as 100.degree. F. but are more prone to formation at the temperature of the processing system and the hydrocarbon fluid at levels of 600.degree. to 1300.degree. F. At these temperatures, coke formation is likely to occur, regardless of the type of hydrocarbon being charged.
One solution to this coking problem is achieved by lowering the reaction severity by means of lowering the reaction temperature. The downside to this method is the resultant decrease in product yield.
The present invention is particularly effective in hydrocarbon processing systems where temperatures reach levels of 600.degree. to 1300.degree. F. where amorphous and filamentous coke are likely to be formed. Amorphous coke is generally produced in systems that operate at temperatures less than 850.degree. F. This type coke generally is composed of low molecular weight polymers, has no definite structure and is sooty in nature. Above 850.degree. F., filamentous coke is generally encountered. This type coke, as the name indicates, takes the form of filaments that appear in some cases like hollow tubes. As opposed to amorphous coke, filamentous coke is not sooty and is hard and graphitic in nature.
Amorphous and filamentous coke formation is customarily found in hydrocarbon processing systems such as delayed coking (operating temperature 900.degree. to 1300.degree. F.); platforming, catalytic reforming and magna forming processes (900.degree. F.); residue desulferization processes (500.degree. to 800.degree. F.); hydrocracking processes (800.degree.-1000.degree. F.); cracking of chlorinated hydrocarbons and other petrochemical intermediates at similar temperatures.
Pyrolytic coke is produced in olefin manufacture where pyrolysis of gaseous feed stocks (ethane, butane, propane, etc.) or liquid feed stocks (naphthas, kerosene, gas oil, etc.) are "cracked" by exposing such stocks to temperatures of from 1400.degree. to 1700.degree. F. to produce the desired olefin.
While various treatments have been proposed to eliminate or reduce filamentous coke formation at 600.degree.-1300.degree. F. temperatures, none have proven as efficacious as the present invention.